The present invention relates to transmission line signal sensing circuits and particularly to telephone disconnect circuits and especially telephone disconnect circuits having means for conserving power supplied via the telephone transmission lines.
A problem with prior art telephone disconnect circuits, in which a signal is transmitted on a telephone line to the disconnect circuit to thereby allow the subscriber telephone device to be disconnected from the telephone line so that the operation of the telephone line can be tested, arises from the fact that the circuits either must be powered from power sources located at the subscriber location, or power is to be supplied over the telephone lines. In the former situation, a power supply must be provided and the subscriber must pay for the expense of supplying power and the power supply adds bulk to the disconnect circuit. In the latter situation, the disconnect circuit and accompanying detection circuitry for detecting the disconnect signal may load down the telephone lines, and often cannot be designed so that sufficient power can be supplied from the ordinarily meager currents available over the telephone lines.